We are talking about the default photos app correct not iphoto or 3rd party?

If so it sounds like a bug in the software these things can happen you have a few options.
1. close all of the apps and power cycle the device
2. if problem persists back up your device. place your phone into DFU mode (with the device off hold the power and home buttons when you see the apple logo release the power button until you see a lightning plug going into the itunes logo) plug the device into your computer and click restore it will download new software and restore your device to your back up.
3. if 2 does not work that means it backed up your bad batch of code and you will have to follow the same steps as 2 but you will have to set up as a new iphone so it is all entirely fresh code. That should solve your problem but it means you cannot use your backed up data so use this as a last resort.